The present invention relates to the high-severity reforming of hydrocarbon stocks to produce highly aromatic reformates which are suitable for use in blending high aromatic content motor gasolines and/or for the production of high purity aromatic petrochemical products. More particularly, the present invention relates to a process for the high-severity reforming of naphtha stocks, using a reforming catalyst which includes a hydrocracking promotor metal to produce aromatic hydrocarbons, and especially to produce aromtic hydrocarbons in heretofore unobtainable yields and with a minimum concentration of non-aromtic mterial present therewith under conditions in which the balance between the vrious reforming reactions is controlled by catalyst modifiction to favor production of aromtic hydrocarbons.
The process of the present invention is particularly efficacious for the production of C.sub.6 to C.sub.8 aromatic hydrocarbons with outstanding purity and yield. As will be understood by those skilled in the art, the term "C.sub.6 to C.sub.8 aromatic hydrocarbons" as used herein refers to aromatic hydrocarbons having 6 to 8 carbon atoms per molecule, and includes such aromatic hydrocarbons as benzene, toluene, and xylene. As also used herein, the term "xylenes" refers to the C.sub.8 aromatic hydrocarbons in a generic sense and includes para-xylenes, meta-xylenes, ortho-xylenes, and ethylbenzenes. Alternatively, while the process of the instant invention is particularly suitable for the production of aromatic hydrocarbons in high purity, it may also be utilized with advantage in the production of highly aromatic gasolines, as will become apparent to those skilled in the art.
Moreover, as further used herein, the term "highly aromatic reformates" refers to those reformates which are of sufficient quality to yield C.sub.7 and/or C.sub.8 aromatic hydrocarbons of commercially acceptable quality directly upon fractional distillation without the necessity for solvent extraction or extractive distillation. Generally such reformates will have a research clear octane value of at least about 100. Accordingly, the present invention contemplates the production of reformates of at least about 100 research clear octane. Reformates of this octane value are highly useful in the production of high purity aromatic hydrocarbons, and are also highly advantgeous for use as blending stocks in the preparation of high octane, lead-free motor gasolines having a high aromatic content.
In the production of aromatic hydrocarbons, it is well known that naphthas contain large amounts of naphthenes and paraffins which can be catalytically reformed to aromatic hydrocarbons, and particularly C.sub.7 and/or C.sub.8 aromatic hydrocarbons under conditions effective for dehydrogenation, isomerization, and dehydrocylicization. It is also well known that platinum group metal based reforming catalysts which include a hydrocracking promotor metal are highly efficacious reforming catalysts. See, for example, U.S. Pat. Nos. 2,848,377 and 4,000,058.
Heretofore, the presence of higher boiling C.sub.8 and C.sub.9 non-aromatic compounds, particularly the C.sub.8 paraffins, in the naphtha feedstock has posed significant obstacles to the production of high purity mixed xylenes in high yields. Likewise, the presence of the higher boiling C.sub.7 and C.sub.8 non-aromatics, particularly the C.sub.7 paraffins, in the feedstock has posed an obstacle to the production of high purity toluene in high yield. In conventional reforming processes, significant quantities of these non-aromatic materials are not converted to aromatics and/or cracked to lower-boiling easily removable compounds. Consequently, reformates produced under conventional reforming conditions contain significant amounts of non-aromatics which cannot be separated from the aromatic material by low-cost separation techniques, such as fractional distillation, but only with great cost and difficulty, such as by solvent extraction or extractive distillation. Accordingly, in order to produce a C.sub.7 and/or C.sub.8 aromatic hydrocarbon product of commercial quality, it is conventional to subject the resultant reformate to a costly solvent extraction or extractive distillation step. Due to the large cost attendant to solvent extraction or extractive distillation, and the additional manpower required therefor, the prior art has sought to develop reforming processes which produce reformates which do not require expensive purification procedures in order to produce an aromatic hydrocarbon product of commercially acceptable quality.
Generally, these prior art processes have involved reforming the naphtha stocks under reforming conditions of high severity in order to crack the paraffins to easily removable gaseous hydrocarbons. In conventional high-severity reforming processes, however, the high severities necessary to produce a reformate having a concentration of unconverted non-aromatics sufficiently low to yield high-purity aromatic hydrocarbons without solvent extraction has also resulted in the cracking of significant quantities of aromatic precursors, with a concomitant decrease in yield of the aromatic product. Hitherto, therefore, conventional high severity reforming processes have been unable to realize the C.sub.7 and/or C.sub.8 aromatic hydrocarbons in satisfactory yields.
One approach to this problem has been to prefractionate the naphtha feedstock into very narrow boiling range naphtha heartcuts in order to exclude the higher boiling non-aromatics, which when reformed would result constituents which contaminate the aromatic hydrocarbon product and which can be removed therefrom only by solvent extraction or other expensive purification. By employing such prefractionations, the amount of difficulty convertible non-aromatic material is reduced, and consequently the reforming process can be operated under less severe reforming conditions, thereby reducing the volume loss resulting from high severity reforming. For example, in U.S. Pat. No. 3,635,815, a naphtha feed fraction is prefractionated into an overhead fraction having an upper endpoint of 270.degree. F. to 275.degree. F. and a bottoms fraction having a higher endpoint. The overhead fraction is then catalytically reformed under reforming conditions of severity sufficient to convert any remaining non-aromatic material to easily removable compounds. The resulting reformate is then subjected to a plurality of fractionation steps to produce a mixture of high-purity C.sub.8 aromatic hydrocarbons.
Similarly, in U.S. Pat. No. 3,499,945, a petroleum naphtha fraction is prefractionated to produce a C.sub.7 naphthene containing heartcut having a distillation endpoint of 175.degree. F. to 220.degree. F. The C.sub.7 naphthene and paraffins containing heartcut is reformed to convert toluene precursors, such as the C.sub.7 naphthenes, to toluene, yielding a reformate which is fractionated to produce a bottoms fraction, boiling above 225.degree. F., rich in toluene. Fractionation and thermocracking of the 225.degree. F. endpoint bottoms fraction then yields a high-purity toluene product.
While the above processes product C.sub.7 and/or C.sub.8 aromatic hydrocarbons of adequate purity, these processes still achieve less than desirable yields. Prefractionation of the naphtha feedstocks into such very narrow boiling range fractions removes significant quantities of C.sub.6 to C.sub.8 romatic hydrocarbon precursors from the conversion process and correspondingly reduces the yield of C.sub.6 to C.sub.8 aromatic hydrocarbons per volume of naphtha feed.
It is also known in the art that the aromatic content of gasolines may be increased by first reforming a naphtha stock under conditions such as to minimize hydrocracking, and then cracking the resulting reformate to convert a large part of the paraffin content to low-boiling olefins. U.S. Pat. No. 2,908,629 describes one such process in which a straight-run naphtha is first reformed by contact with a dual-function dehydrogenation catalyst under conditions which minimize hydrocracking, and then cracked by contact with a conventional cracking catalyst to convert a large portion of the paraffin content to lower boiling non-aromatics. However, while this process reduces naphthene cracking, it requires the operation of an additional process unit and the thermal cracking results in some reduction in potential aromatic hydrocarbons. Moreover, in the upgrading of gasolines, it is desirable to convert the heavy paraffinic material to high octane branched paraffins. The formation of reformates containing a high percentage of desirable isoparaffins is incompatible with the production of reformates suitable for the preparation of high-purity aromatic hydrocarbons, and consequently would preclude the production of the aromatic compounds in pure form without extensive subsequent purification. Accordingly, this process is less satisfactory for the production of high-purity C.sub.6 to C.sub.8 aromatic hydrocarbons.
In view of the great demand for commercial purity C.sub.7 and/or C.sub.8 aromatic hydrocarbons, and in view of our ever declining supplies of petroleum, the low yields and/or aromatic content of reformates obtained with the prior art reforming processes renders their use undesirable. Accordingly, there exists a great need in the art for a process for the manufacture of highly aromatic reformates having a low concentration of difficultly separable non-aromatic compounds in high yield.